Adequate regeneration and new growth of small blood vessels is essential to the survival and healing of tissues exposed to therapeutic doses of ionizing radiation, especially when surgical trauma is superimposed. The work proposed here will examine the response of microvessels to graded doses of ionizing radiation, when microvascular growth is induced either before or after irradiation. The model system is inflammatory corneal neovascularization, induced by silver nitrate cauterization. The growth response is evaluated by measuring the length and density of new vessels, which invade the previously avascular cornea in radial fashion toward the central lesion. Terminal perfusion of colloidal carbon renders the vessels opaque and facilitates measurements. To assess proliferation of vascular endothelial cells, 3H-thymidine is injected before sacrifice and autoradiographs of plastic sections are prepared. Electron microscopy is used to evaluate the structural integrity of the vessels. The radiation response will be assessed by quantitation of actual vessel formation and 3H-thymidine labeling at 2, 4 and 7 days after graded single doses of x-rays followed immediately by cauterization. Slow radiation repair capability will be assessed by increasing the interval between irradiation and induction of neovascularization to 60 days. Elkind-type repair will be assessed using split-doses at intervals of 1-20 hours, with induction of neovascularization just after the second dose. This experiment will also be done on rapidly proliferating endothelium, by applying split-doses after induction of neovascularization.